Methylamine dehydrogenase (MADH), a metabolic enzyme found in methylotrophic/autotrophic bacteria, contains a quinone cofactor, tryptophan tryptophylquinone (TTQ), derived from the post-translational modification of two Trp residues in the protein. The maturation of MADH involves at least 4 other proteins, and we have begun to characterize one of these, MauG. It is a highly unusual di-heme enzyme responsible for the completion of TTQ synthesis. The natural substrate for MauG (preMADH) is a 119-kDa protein precursor of MADH with a partially formed cofactor. MauG catalyzes a six-electron oxidation to complete TTQ biosynthesis, using three moles of either molecular oxygen or hydrogen peroxide as the second substrate. The two hemes of MauG act as a single redox unit, and the catalytic reaction involves an unprecedented high-valent di-heme intermediate that is unusually stable. The high-valent species is an FeV equivalent consisting of one FeIV=O heme, with the other in an FeIV oxidation state. During the previous funding period we have solved the crystal structure of MauG in complex with preMADH. This has been achieved to a resolution of 2.1 , and these crystals can support catalytic turnover to form TTQ without loss of diffraction. The MauG-preMADH structure, along with other recent data from collaborating labs, has been full of surprises. The structure indicates there are no major structural rearrangements during catalysis and that long-range inter-protein electron and radical transfer occurs in this system. During catalysis a very stable MADH protein radical is also produced. Now we wish to move the crystallographic studies, complemented by mass spectrometry and single crystal spectroscopies, beyond what was envisioned during the previous funding period, and make specific discoveries about oxygen activation by a previously unknown high-valent iron intermediate, mechanisms of oxidative modification to specific amino acid residues within a protein, and stabilization of highly reactive oxygen species (ROS) and radicals. Fundamentally these studies will give molecular insight into inter-protein electron transfer central to metabolis, and the control and outcomes of oxidative damage by radicals and ROS that are associated with many disease states. Thus protective control of electrons, radicals and ROS within the protein matrix is a key component of human health.